Overvoltage protection is typically provided by voltage dependent resistors, such as Schottky diodes based on SiC, or varistors, based on ZnO, which work on solid state principles related to grain boundary conduction. The present invention is related to filters suitable for removing high frequency electromagnetic interference (EMI) from electronic signals whilst protecting the component and associated circuitry from voltage spikes or surges. More specifically, the present invention is related to multi-layered ceramic filters.
The most popular type of voltage dependent resistors, or varistors, are based on zinc oxide doped with other elements to control grain boundary conduction. These devices depend on their non-linear I-V behavior to suppress transient voltage surges. However, there are some significant compromises that result from their use. Voltage permanently applied to the varistor must be carefully limited to avoid excessive power dissipation. Since they often have a negative temperature coefficient of resistivity a runaway condition can easily be initiated. Subjecting varistors to electric fields can change the characteristic and result in an increase in current and power dissipated as heat, degrading performance.
Filtering of electronic signals is also a widely practiced art typically practiced with filters such as discoidal filters, Pi filters, T filters, LC filters and the like. In general, the type of filter is chosen based on the filtration needs and one of the most common needs is to filter EMI noise from signals.
Discoidal filters are commonly used where the discoidal filter is placed on the input line and contacted to ground. Placement of the filter is typically a manual operation which is not readily automated thereby significantly increasing the cost of devices containing discoidal filters. In the most demanding applications, such as on the leads of medical implantable devices, the leads must be hermetically sealed, typically in a can, and the can is grounded.
Surface mount filters are available which have some advantages over discoidal filters. Surface mount filters are typically 4 terminal capacitors with input and output terminals perpendicular to each other. This design dictates the relative location of the ground and signal traces which places a constraint on the circuit designer. The perpendicular design also complicates efforts to miniaturize the overall circuit size. As the components become smaller the distance between the signal and ground must be decreased to accommodate the filter terminations. Unfortunately, this increases the potential of arcing, particularly in high voltage applications. The perpendicular orientation also makes the use of slotted boards impractical and electromagnetic radiation interference mitigation is not typically available.
Pi, T, and LC filters are widely used in either feed-through or surface mount configurations. The surface mount filters are preferred over feed-through when considering further miniaturization even though the electromagnetic radiation interference mitigation is superior in the feed-through configurations. The surface mount configurations comprise a magnetic inductor and a ceramic filter and integrating these components is a significant challenge. The magnetic inductor is typically ferrite or ferromagnetic materials which are not compatible with the materials used in multi-layered ceramic capacitors due to the sintering requirements of ceramic.
There has been an ongoing desire for a filter which is small in dimension yet which does not increase complexity in circuit design. There has also been an ongoing desire for a filter which is easily manufactured, is preferably surface mountable or embeddable, and which can be adapted to various circuit requirements.
There has also been an ongoing desire for an overvoltage protection device which does not have the deficiencies of the prior art.